It is known that free sulfur can be recovered from gases of widely varying hydrogen sulfide content. The problem to which the present invention is directed concerns either straight-through or split-flow plants in which the feed rate of gas to the sulfur recovery unit is subject to wide variation, e.g., from 100 percent down to less than 50 percent of the design feed rate.
With acid gas having, for example, a hydrogen sulfide concentration range of about 45 to 100 mol percent, all of the gas -- along with the proper amount of air -- is fed to a burner, the combustion products from which exhaust into a furnace and, in turn, are discharged into a suitable waste heat boiler. This is the so-called straight-through operation. The furnace acts as a noncatalytic reactor converting from about 40-60 percent of the hydrogen sulfide in the feed to free sulfur. The product sulfur is generally condensed from the boiler effluent before passing to the first reactor. The sulfur thus removed allows for a lower reactor feed temperature which improves yields without incurring catalyst deactivation by sulfur deposition.
Many sulfur recovery units have used reheat gas extracted from the waste heat boiler to preheat feed gas to the catalytic reactors. This practice, which is referred to as "bypass reheat," involves withdrawing a portion, e.g., 10-20 percent, of the boiler effluent at a temperature of from about 900.degree. to 1200.degree. F. and mixing it with cooled (325.degree.-375.degree. F.) gas from the condenser handling the balance of the boiler effluent, to give a reactor feed having a temperature of the order of 425.degree.-450.degree. F. This is a relatively inexpensive reheating method which is generally satisfactory, at least for the first reactor in a two-reactor plant (and for the first reactor or first two reactors in a three-reactor plant) when an exchanger is used to preheat feed gas to the final reactor. The optimum reheat gas temperature is fixed principally by the following considerations. Higher temperatures reduce the amount of reheat gas required and increase sulfur recovery efficiency since bypassing of elemental sulfur to the catalytic reactor is decreased. On the other hand, higher temperatures require more expensive materials for construction of the reheat gas piping and valve. The optimum design temperature is usually in the range of 900.degree. to 1200.degree. F.
Once the optimum reheat gas temperature and materials of construction have been selected and the equipment has been installed, it is desirable to maintain this temperature substantially constant at all times. In the past this has not been possible because the reheat gas temperature decreases when the plant feed gas rate is below design. When the gas flow rate through the boiler declines, the temperature of the reheat gas is lowered because of more effective cooling in the waste heat boiler. For example, when the design temperature is 1200.degree. F., the temperature is about 900.degree. F. at 40 percent of design feed rate. In addition, the temperature of the condenser effluent gas to be reheated declines. It can be seen that with this type of reheating procedure if the feed rate is materially decreased a greater proportion of the boiler effluent must be used for preheating purposes in order for the reactant gases to be raised to the proper temperature level prior to entering the reactor. The boiler effluent used as reheat gas contains free sulfur in vapor form. The presence of a substantial concentration of sulfur in this reheat gas interferes with reaching favorable equilibrium conditions in the reactor, and accordingly, results in a loss in yield as the proportion of reheat gas is increased. As the plant feed rate declines below 40 percent, the reheat gas temperature continues to decrease and point is eventually reached where it is impossible to even maintain the desired reactor inlet temperature. Finally, for muffle furnace plants when heating the plant up on fuel gas with inert gas to moderate the flame temperature, the inert gas is costly and usually has a limited rate. Thus, it may be impossible to obtain a high enough reheat gas temperature to adequately heat the reactors.
To overcome the above adverse effects my invention provides an improved process which results in a substantially constant temperature of the reheat gas as the plant feed rate declines.